Serum osmolality may fluctuate with neurohormonal activation and in response to certain therapeutics in patients with heart failure (HF). The clinical relevance of osmolality in patients with HF has not been defined. In this post hoc analysis of the Efficacy of Vasopressin Antagonism in Heart Failure: Outcome Study with Tolvaptan trial, we analyzed serum osmolality measured at discharge in 3,744 patients hospitalized for HF and reduced ejection fraction (EF ≤40%). Median follow-up was 9.9 months. The association between discharge osmolality and all-cause mortality (ACM) and composite cardiovascular mortality or HF hospitalization was nonlinear; and thus, patients were divided into low (≤284), normal (285 to 300), and high (≥300 mOsm/kg) osmolality. Median serum osmolality at discharge was 297 (290 to 304) mOsm/kg. Patients in the low osmolality group (n = 454,12.1%) were more likely to be younger, men, have lower rates of hypertension, coronary artery disease, chronic kidney disease, diabetes, and have lower serum sodium, creatinine, systolic blood pressure, and EF (all p <0.001). Low discharge osmolality was associated with higher ACM (low 29.3%; normal 23.6%; high 25.2%; p = 0.04) and the composite endpoint (low 45.6%; normal 39.3%; high 41.8%; p = 0.04). After risk adjustment, a 15 mOsm/kg decrease in osmolality was predictive of higher ACM (hazard ratio 1.61, 95% CI 1.19 to 2.17) and the composite endpoint (hazard ratio 1.37, 95% CI 1.06 to 1.75) in the low osmolality group. These associations were not seen in patients with normal or high osmolality. Interaction analyses for tolvaptan treatment were nonsignificant (p >0.4). In conclusion, low discharge serum osmolality was independently predictive of worse postdischarge mortality and readmission. Further study is required to clarify the clinical utility of serum osmolality in hospitalized patients with HF.